When the size of an object in an image is illustrated by means of the image, a reference object needs to be placed in a scene where the image is photographed. For example, a piece of A4 paper is used as the reference object to be contrasted with an air purifier so that a viewer may have an intuitive cognition of the size of the air purifier. However, objects may be photographed at different distances and thus the photographed objects may occupy different numbers of pixel points on an image sensor of a camera. Therefore, it is unable to conveniently learn about the real size of the photographed object by means of the reference object t in the image.